Nitrogen-containing water pollutants may be organo-nitrogen compounds or inorganic nitrogen-containing salts, particularly ammonium salts and nitrate or nitrite salts. Often these two types of pollutants are found together in a wastewater. The removal of these substances from wastewaters poses a formidable treatment task.
The toxic characteristics of many of these nitrogen-containing substances makes biological treatment impossible or impractical due to the large dilution factors required to allow some form of biological treatment. Various so-called advanced oxidation processes, such as ozone, peroxide or UV treatment, alone or in combination, may be ineffective for all the nitrogen-containing compounds in a particular waste. Multiple treatment steps may be employed but this is a costly alternative which can be avoided.
The Stamicarbon B.V. patent, GB 1,375,259 discloses the decomposition of ammonium nitrate solutions at elevated temperatures and pressures. The examples given are for the treatment of a caprolactam waste with ammonium nitrate as the oxidation agent with or without air present. The treatments were performed at 250.degree. C. to 300.degree. C. and showed excellent removal of COD from the waste. Another example described treatment of sludge containing a Chemical Oxygen Demand (COD) of 20.7 g/l with 100 g/l ammonium nitrate at 300.degree. C. for 2 hours to give 80% COD removal. Nitric acid was also used as the oxidizing agent in autoclave experiments.
German patent application 2740536 by Ciba-Geigy describes a two step process for wastewater treatment. In the first step wet oxidation converts nitrogen-containing compounds to ammonia, and in the second step a solution of sodium nitrite is added to the ammonia-containing wastewater to decompose the ammonia therein according to the equation: EQU (NH.sub.4).sub.2 SO.sub.4 +2NaNO.sub.2 .fwdarw.Na.sub.2 SO.sub.4 +2N.sub.2 +4H.sub.2 O
Harada et al. in U.S. Pat. No. 4,654,194 disclose the use of a noble metal catalyst supported on a titania carrier to decompose ammonium nitrate in solution. Examples were carried out a 250.degree. C. for 60 minutes to give 50-99% decomposition of both ammonium nitrate and nitrite without air present. Further examples are given where an organic (phenol) was added with 0.2 times the required amount of oxygen. Decomposition of both ammonium nitrate and phenol was found.
Osaka Gas Co. has filed a number of Japanese patents concerning decomposition of ammonium nitrate wastewaters. One group, JP 61 257,292 and JP 61 257,291, disclose wet oxidation of ammonium nitrate wastewaters with 1.0 to 1.5 times the stoichiometric oxygen required for ammonia decomposition, at pH 3-11.5 at 100.degree.-370.degree. C. with a supported noble metal catalyst. Another group, JP 4,059,094, JP 4,061,987 and JP 4,200,692, disclose adding organic substances to an ammonium nitrate wastewater and pyrolysis of the mixture at 100.degree.-370.degree. C. in the absence of oxygen with a supported noble metal catalyst, then wet oxidizing the effluent at similar conditions with oxygen-containing gas.
Cox et al. in U.S. Pat. No. 5,118,447 disclose a process for thermochemical nitrate destruction where an aqueous solution of nitrate or nitrite is contacted with a stoichiometric amount of formic acid or formate salt, depending upon pH. The mixture is heated to 200.degree. C. to 600.degree. C. in the liquid phase to form elemental nitrogen and carbon dioxide. The reaction may be carried out over the pH range of 0-14.
Fassbender, in U.S. Pat. No. 5,221,486, discloses another denitrification process where the types of nitrogen compounds present in a waste stream are identified and quantitated. The oxidized and reduced forms of nitrogen are balanced by adding an appropriate nitrogen containing reactant such as ammonia or a nitrite or nitrate compound, and then the mixture is heated to 300.degree. to 600.degree. C. under pressure to obtain the denitrification reaction.
Wet air oxidation is a well known treatment process for the removal of carbonaceous COD and BOD from wastewaters. High degrees of destruction of toxic and hazardous compounds in wastewaters by wet oxidation are well documented. The process involves contacting a wastewater with an oxygen-containing gas at elevated temperatures and pressures to oxidize pollutants. Temperatures up to the critical temperature of water, 373.degree. C., are employed, with corresponding pressures sufficient to maintain water in the liquid phase. The wet oxidation occurs in the liquid phase without the formation of gaseous oxides of nitrogen or sulfur, as occurs in high temperature gas phase combustion. Most carbonaceous material is converted to carbon dioxide, although some low molecular weight oxygenated species, such as acetic acid, are usually produced. The nitrogen present in organo-nitrogen compounds generally reports as ammonia, nitrate or nitrite in wet oxidation effluent. This liquid often requires additional treatment before discharge to the environment. Applicants have discovered a process which combines wet oxidation of wastewaters at moderate temperatures and pressures, and catalytic decomposition of nitrogen species in the wet oxidation effluent, also at moderate temperatures and pressures, to remove nitrogen and produce a highly treated effluent.